The creation of a ‘coastal crust’ of defended and obstructed possible landing sites was supplemented by a series of ‘stop lines’, exploiting natural barriers enhanced by obstacles and strongpoints, to delay an invasion force and protect the central areas. The most significant of these lines was the GHQ line, running from Edinburgh and Bridgwater to London. In Wales, successive rivers were chosen, including the Rivers Wye, Usk, and Neath. The limited number of crossings were protected by pillboxes, and arrangements were made for the demolition of unwanted bridges.
The OB at Cilybebyll, in woodland 100m northwest of Plas Cilybebyll house (SN 750 043: it is on private land), survives in good condition (fig. 1); the author was invited to investigate the site by the landowner. On the surface, the OB is visible as a slightly-raised area; at the north end, the shafts are visible as brick features, no longer hidden by a trap door. There is little doubt that when constructed it would have been concealed from all but the closest scrutiny (fig. 2).
The interior now has a flooded floor. A winch arrangement on the entrance wall survives; this may have been attached to an aerial for radio communications or to the trap door. An alcove on the east side of the door may have been for the radio equipment.
This site is due to be surveyed in February 2013
[text taken from http://www.coleshillhouse.com/cilybebyll-auxiliary-unit.php]
